The information and assistance an effective counselor provides can have considerable and long-lasting benefits for their students, boosting college outcomes years after they graduate high school. Improving access to effective counselors may be a simpler and more cost-effective way to increase educational attainment than improving access to effective teachers. This study focuses on Massachusetts, where a quasi-random assignment process was used to assign many high-school students to counselors based on the student's last name. The study looks at the outcomes of each counselor's students to determine individual effectiveness, as well as at counselors' experience, educational background, and other characteristics to identify the attributes of those who are most effective. While advocacy groups often focus on reducing counselor caseloads as a means of boosting their impact, the study suggests that making counselors more effective may be a better goal. That assumes there is a straightforward way to improve effectiveness, which is admittedly a matter for future research. There is one simple and inexpensive way to put these findings to work right away, however: increase the diversity of the counselor workforce, especially in schools serving large numbers of minority students. Counselors are and should be considered an important resource for addressing educational inequities and increasing educational attainment.